1. Field of the Invention
This invention relates to digital transmission systems, and more particularly to a correlative technique which permits transmission of digital information at speeds significantly above presently achievable rates in a band limited channel.
2. Description of the Prior Art
The continuing demand for the rapid transmission of data has created a requirement for new data transmission techniques. However, in systems of which applicant is aware, the increase in transmission rate is achieved only at the expense of unacceptable equipment complexity, and hence greater cost, or in poor performance, relative to a binary system. One example of known band width compression techniques which permits transmission of more than one bit of information in a Nyquist interval is the quaternary baseband system, which compresses the bandwidth by a factor of two relative to binary. Here, serial binary data, represented by 0 and 1, is converted at the transmitter into four levels, each of which represents two of the original binary digits. In Gray code, the successive levels would represent 00, 01, 11 and 10. For example, the use of codes where successive levels differ only by one bit, such as the Gray code, is preferred because the difference of interpretation between adjacent levels causes only one of the digits to be in error. For example, if the level is actually 1, in which the two digits of the quaternary system are 01, an interpretation of the level at the receiver as 0 due to distortion caused by transmission impairments would result in an output sequence of 00, thus causing only a single error.
Similar compression techniques may be used in carrier transmission using AM, FM, phase modulation, etc. Where carrier is used and compression is required, phase modulation can be used, for example, with n phases. The number of phases could be 4, 8, 16 or 32 for a practical system in which the total number of phase positions is a power of two. 4-level or 4-phase systems, which permit the transmission of one bit of information per cycle of available bandwidth in double side band carrier transmission, have been used commercially. This system, then, has a data transmission rate twice that of a binary system transmitting over the same band-limited channel.
The old and well-known vestigial side-band transmission technique has gained popularity in recent years as a means of compressing the bandwidth for high-speed data transmission. However, the success of this system depends on synchronous detection which requires that the carrier be regenerated with the correct frequency and phase at the receiver. The frequency can be quite accurately regenerated by the use of pilots. However, the phase must be recovered from the modulated signal and the characteristics of the vestigial signal makes accurate recovery of signal phase quite difficult. The accuracy with which the carrier is regenerated directly affects the permissible rate of transmission and error rates.
In comparing bandwidth compression transmission techniques with a binary system, not only is the complexity, and hence cost, of the equipment considered but the error performance relative to that of the binary system must also be evaluated. Error performance is most often established in terms of the noise penalty suffered by the higher speed system. Many factors affect the noise penalty, but the approximate value is assumed to be dependent upon the ability of the system to interpret a particular amplitude level or its equivalent. As shown in applicant's article entitled, "Correlative Level Coding for Binary-Data Transmission", IEEE Spectrum, Feb. 1966, Page 107, the approximate noise penalty relative to a binary system in dB, is 20 log.sub.10 (b - 1), where b is the number of levels. For quaternary AM system, the noise penalty is approximately 9.5 dB, and while the noise penalty for a 4-phase system is somewhat less than that of the AM quaternary, it suffers from the cost and complexity of the equipment required for proper recovery of the transmitted information.
The foregoing brief review shows the desirability of increasing the transmission rate without squaring the number of levels or phases for each doubling of the bit rate, as in the case for the multilevel techniques discussed above. It is also desirable to have a technique whereby each level or phase separately identifies the original binary data bits without regard to the past history of the waveform. These desirable criteria are found in the correlative techniques described in the aforementioned IEEE Spectrum article, and the duobinary correlative technique mentioned therein are described in greater detail in applicant's U.S. Pat. Nos. 3,234,465 and 3,238,299. Another correlative technique, which is referred to as the "Orthogonal Correlative Technique" is disclosed in applicant's U.S. Pat. No. 3,515,919, and assigned the assignee of the present application. These correlative techniques require fewer levels than the prior art multilevel systems and have the further desirable feature that each level separately represents one ordinary binary digit. An additional feature of applicant's earlier correlative techniques is that line signal follows predetermined rules which permit error detection without the need for adding redundant digits.
Digital systems using the duobinary or orthogonal correlative techniques rarely permit doubling or quadrupling, respectively, the data rate with a minimum of equipment complexity in cost. A nonbinary correlative technique is disclosed in applicant's U.S. Pat. No. 3,601,702. This nonbinary technique is an improvement over the other prior art correlative techniques in that it readily increases the data rate to eight times, or more, that of a binary system. Thus, if a binary system could transmit data at 1200 bits per second, a duobinary system could transmit 2400 bits per second, and an orthogonal correlative system could transmit 4800 bits per second and the systems in accordance with the present invention could transmit 9600 bits per second, or more, in the same bandwidth.
The most serious and difficult problem in practical correlative systems of the type described hereinabove which have more than three levels, is the generation of a high quality signal with equal spacing between levels. This is very essential since the undesirable intersymbol interference increases rapidly with an increasing number of levels. The least number of levels in a correlative signal is three. Here the horizontal width of the open eye, of a standard eye pattern, is about 35% of the bit duration. When the number of levels is increased to 7, the horizontal width of the open eye decreases to about 12% of the bit duration. In this case, it is essential, to provide a high accuracy in level generation as well as the most efficient method of encoding.